For example, permanent magnets, such as rare-earth magnets, have a high energy density and are often required to reduce a size of an electric motor. However, geological locations of the sources of rare-earth metals are limited. Therefore, it is desirable to minimize the amount of use of the permanent magnets. JP2010-259304A (corresponding to US2010/0133939A1) teaches a rotor that has a consequent-pole structure in an axial intermediate portion of a rotor core of the rotor to limit or reduce the amount of permanent magnets. Specifically, in the consequent-pole structure of this rotor, a plurality of projections radially outwardly projects from a boss of the rotor core, and each of the permanent magnets is circumferentially held between corresponding adjacent two of the projections. The permanent magnets are held by a cylindrical tubular cover, which is fitted to a radially outer surface of the rotor core, so that radially outward displacement of the permanent magnets caused by a centrifugal force of the rotating rotor core can be limited.
In the rotor of JP2010-259304A (corresponding to US2010/0133939A1), a radius of curvature of a radially outer surface of each permanent magnet and a center of curvature of the radially outer surface of each permanent magnet are designed to coincide with a radius of curvature of a radially outer surface of each projection and a center of curvature of the radially outer surface of each projection, respectively. Therefore, depending on a manufacturing accuracy, the following situation may possibly occur. Specifically, in a case where the manufacturing accuracy is reduced, i.e., is deteriorated, although the radially outer surface of each of the projections can entirely contact the cover, the contact between the permanent magnets and the cover may possibly become insufficient due to the reduced manufacturing accuracy. Therefore, in such a case, the sufficient holding force for holding the permanent magnets with the cover cannot be achieved. Furthermore, due to the small contact surface area between each permanent magnet and the cover, the compression stress, which is exerted on the permanent magnet, may possibly become large to cause a damage of the permanent magnet.
Furthermore, in the case where the radially outer surface of each of the projections entirely contacts the cover, a frictional resistance, which is exerted at the time of press-fitting the cover to the projections and the permanent magnets, may possibly become large to cause application of an excessive press-fitting load. Particularly, in a case where the rotor core is a laminated core, which includes a plurality of steel plates that are stacked one after another, the radially outer surface of each projection of the rotor core, which contacts the radially inner surface of the cover, is formed as a press-cut surface that is cut with a press machine. Therefore, in such a case, the frictional resistance at the time of press-fitting the cover to the rotor core may not be uniform due to influences of burrs and/or shear droop at the press-cut surface. Thereby, in such a case, the press-fitting load is further excessively increased. Therefore, in the case where the press-fitting load becomes excessive, the size of the press-fitting apparatus needs to be increased. Thus, the manufacturing costs may be increased. Also, buckling of the cover may possibly occur during the press-fitting process of the cover.